The measurement of fluid flow is becoming increasingly more important in the application and control of many processes, as well as in the research laboratory. The ideal fluid flowmeter will not load down the system in which flow is being measured, has absolute accuracy, and can function as a primary standard for gas or liquid flow measurement and calibration. One type of flowmeter and calibrator for measuring gas flow is the soap film flowmeter. In its basic form, a soap film is generated from a soap solution and is propelled through from one end of a flowtube to the other by the gas flow under measurement. By timing the rise of the soap film between designated positions along the flowtube, the gas flow rate can be readily computed. The advantage of this system is that the soap film is virtually massless, and the seal between the soap film and the tube wall is essentially leakproof, thereby providing a measurement of close to ideal accuracy. The disadvantages are primarily due to the expediency of a soap solution to form the soap film, which makes the device somewhat unwieldy for field operation. Moreover, the humidity of the sampling media must also be controlled.
Another type of flowmeter is the positive displacement piston flowmeter. The use of a free piston movable in a close-fitting tube as a flow rate device has been known for over fifty years. In general, most conventional flowmeters using a free piston mounted in a precision bore glass tube rely on a mercury-wetted, frictionless seal between the piston and the bore. A system which consists of a piston whose coefficient of expansion is similar to the precision glass bore tube to maintain an especially tight seal with exceptionally low leakage is very well known and has been used as a damper. The advantages of a piston-type flowmeter is :ts simplicity of operation and practicality for use in all types of field conditions. However, it is highly sensitive to condensation and contamination from dirt and dust. The presence of dirt or dust in the flowtube will vary the differential pressure requirements required to drive the piston and affect its measurement accuracy. In fact, the tolerance between the piston and cylinder for the precision bore dashpot is so tight that dust or dirt can totally inhibit operation. Heretofore, this mandated sophisticated filtering systems which substantially increases size, manufacturing costs and places an additional load on the system. PG,4